(activite solaire) - arxiv.org e-print archive · commission 10 solar activity (activite solaire)...

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5Transactions IAU, Volume XXIXAProc. XXVIII IAU General Assembly, August 2012Thierry Montmerle, ed.

c© 2015 International Astronomical UnionDOI: 00.0000/X000000000000000X

DIVISION II

COMMISSION 10 SOLAR ACTIVITY

(ACTIVITE SOLAIRE)

PRESIDENT Carolus J. Schrijver

VICE-PRESIDENT Lyndsay Fletcher

PAST PRESIDENT Lidia van Driel-Gesztelyi

ORGANIZING COMMITTEE Ayumi Asai, Paul S. Cally,

Paul Charbonneau, Sarah E. Gibson,

Daniel Gomez, Siraj S. Hasan,

Astrid M. Veronig, Yihua Yan

Abstract. After more than half a century of community support related to the science of“solar activity”, IAU’s Commission 10 was formally discontinued in 2015, to be succeeded byC.E2 with the same area of responsibility. On this occasion, we look back at the growth of thescientific disciplines involved around the world over almost a full century. Solar activity andfields of research looking into the related physics of the heliosphere continue to be vibrant andgrowing, with currently over 2,000 refereed publications appearing per year from over 4,000unique authors, publishing in dozens of distinct journals and meeting in dozens of workshopsand conferences each year. The size of the rapidly growing community and of the observa-tional and computational data volumes, along with the multitude of connections into otherbranches of astrophysics, pose significant challenges; aspects of these challenges are beginningto be addressed through, among others, the development of new systems of literature reviews,machine-searchable archives for data and publications, and virtual observatories. As customaryin these reports, we highlight some of the research topics that have seen particular interest overthe most recent triennium, specifically active-region magnetic fields, coronal thermal structure,coronal seismology, flares and eruptions, and the variability of solar activity on long time scales.We close with a collection of developments, discoveries, and surprises that illustrate the rangeand dynamics of the discipline.

1. Historical context

The IAU was founded in 1919, with the first General Assembly occurring in 1922 in Rome,Italy. The standing commissions that focused on what we nowadays capture under the term“solar physics” evolved over the following decades. A “Commission 10” was instituted by 1922,but under the title of “Solar Radiation”. By 1935 that had been changed to “Sunspots andsunspot numbers”, and by 1952 it had transitioned to “Photospheric phenomena”. In parallelto Commission 10 there were the other solar-oriented Commissions 11 through 15 dealing withsubjects such as the solar atmosphere, eclipses, rotation, and spectroscopy (with, as noted inthe 1961 report by Commission 10, a “lack of clear demarcation lines among the [three] solarcommissions”). It was not until 1961 that “Solar Activity” became the title of Commission10, lasting until its transition into Commission C.E2 in the overall reorganization of the IAU’sstructure in 2015.

The scientific discipline focusing on solar activity continued to grow after Commission 10settled on its final name, seeing among the many activities of its community the launch of adedicated journal “Solar Physics” in 1967 and a marked advance in access to solar corona and

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http://arxiv.org/abs/1510.03348v1

2 DIVISION II COMMISSION 10

Table 1. Overview of Commission 10 leadership and triennial reports (as available in ADS)from 1961 onward, i.e. for the period that C10 operated under the banner of “Solar Activity”or “Activite Solaire”. Reports flagged with an asterisk appear not to be available on line.

Years President and Vice President ADS bibcode

1961-1964 Severny, A. B. , Ellison, M. A. *1964-1967 Svestka, Z. , Jefferies, J. T. *1967-1970 Svestka, Z. , Jefferies, J. T. 1970IAUTA..14...71S1970-1973 Jefferies, J. T., Kiepenheuer, K. O. 1973IAUTA..15...75J1973-1976 Kiepenheuer, K. O., Newkirk, G. A. *1976-1979 Newkirk, G., Bumba, V. 1979IAUTA..17b..11N1979-1982 Bumba, V., Tandberg-Hanssen, E. 1982IAUTA..18...55B1982-1985 Tandberg-Hanssen, E., Pick, M. 1985IAUTA..19...57T1985-1988 Pick, M., Priest, E. R. 1988IAUTA..20...55P1988-1991 Priest, E. R., Gaizauskas, V. 1991IAUTA..21...53P1991-1994 Gaizauskas, V., Engvold, O. *1994IAUTA..22...53G1994-1997 Engvold, O., Ai, G. *1997IAUTA..23..121E1997-2000 Ai, G., Benz, A. O. *2000-2003 Benz, A. O., Melrose, D. B. *2003-2006 Melrose, D. B., Klimchuk, J. A. 2007IAUTA..26...75M2006-2009 Klimchuk, J. A., van Driel-Gesztelyi, L. 2009IAUTA..27...79K2009-2012 van Driel-Gesztelyi, L. , Schrijver, C. J. 2012IAUTA..28...69V2012-2015 Schrijver, C. J., Fletcher, L. (this report)

inner heliosphere with the space-based Apollo Telescope Mount on Skylab in 1973. At present, wehave ground- and space-based observatories looking at the Sun and innermost heliosphere fromdifferent perspectives, in a range of wavelengths, and probing the Sun’s internal dynamics usinghelioseismology. But although these observatories provide a wealth of information and insightinto the workings of our neighboring star, we struggle to provide the research community witha comprehensive view of the phenomena captured under the term “Solar Activity”. That iscertainly not a new challenge: for example, in the 1970-1973 report, then President Jefferiesof C10 notes that “We believe that the major direction in which priority should be placed tofacilitate the understanding of solar activity lies in the provision of space and ground basedobservatories specifically designed to complement each other’s capabilities.”

The early reports of Commission 10 published in the “Reports on astronomy” could high-light many of the key developments in the field overall. With the rapid growth of the disciplinefrom the 1940s through the 1970s that quickly became impossible. From the middle of the20th Century onward, severe selections had to be made in order for the task of the writing of aprogress report to remain feasible. The need for such major down-selects on what to cover clearlyconcerned the members of the Organizing Committees of the Commission (the presidents andvice-presidents are listed in Table 1). For example, in the report on the period 1967-1970, theCommission President Zdenek Svestka apologizes for the “fairly severe” selection made in theCommission’s overview with 32 pages of text as “only about one third of all published paperscould have been mentioned in the references”. By 2015, with some 6,000 papers appearing pertriennium (as discussed in the next section), simply reading all published papers is too challeng-ing a task. Even with severe selections applied by the Organizing Committee of Commission 10,the 1967-1970 report was still deemed too long: John Jefferies notes in the subsequent reportfor 1970-1972 that the General Secretary requested that the reports should “concentrate on themore important developments” (which led to 34 pages of text in that cycle). The most recentreports condense the increased number of publications into 10 to 20 pages of text, selectinghighlights only in the view of the members of the Organizing Committee.

For this report on 2012-2015 we would face a similarly daunting selection task. Instead,we chose to focus on three aspects of our community: the health of the community itself, newdevelopments in the areas of instrumentation and IT infrastructure, and a discussion of scientificdevelopments based on citations within the community and impressions of the OC members ofC10.

SOLAR ACTIVITY 3

Refereed publications / year

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Figure 1. Number of refereed publications per year with abstracts focusing on Sun orheliosphere (as returned by ADS) from 1911 through 2014.

Unique authors / year

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Figure 2. Number of unique authors for each year in publications per year with abstractsfocusing on Sun or heliosphere (as returned by ADS) from 1911 through 2014.

2. Trends in the research community and its publications

As C10 transitions into C.E2 we take the opportunity to review the community’s size andpublication activity. For this, we use the tools provided by the Astrophysics Data System (ADS†),which enables searches over all the major trade publications in astrophysics in general. Wereviewed the number of refereed publications per year going back over a century, and quantifiedthe population of active researchers and their publication productivity.

The study of phenomena related to “solar activity” often involves other aspects of the Sun(such as internal dynamics, dynamo, or surface field patterns) and they are obviously not limitedto the Sun but drive phenomena throughout the heliosphere. We therefore do not attempt aseparation by research disciplines along the somewhat arbitrary dividing lines between the IAUCommissions in what was Division II and is now Division E‡, all the more so because of theshifts in focus of the Commissions related to solar physics since their inception after 1919as noted above. Consequently, we searched ADS for abstracts of refereed publications in the“Astronomy” database, either mentioning the Sun or heliosphere or their synonyms. We filterout at least many of the papers that do not deal with Sun/heliosphere that come into thesearch results because their abstracts include, for example, a unit like “solar mass”; to do so, we

† URL: http://adsabs.harvard.edu/abstract service.html‡ See http://www.iau.org/science/scientific bodies/divisions/


h-index distribution

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Figure 3. Distribution of H indices for all authors publishing on Sun and heliosphere in 2014,based on ADS citation counts.

N papers / N scientists

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Figure 4. Number of refereed publications per author with abstracts focusing on Sun orheliosphere (as returned by ADS) from 1911 through 2014.

exclude abstracts that contain one or more of the following words or word groups: cluster, dwarf,extrasolar, galaxy, gravitational, ice, kpc, solar system, stellar, binary, sunset, sunrise, eclipse,solar cell, solar occultation, interstellar medium, and supernova. Sampling the returned titlesand abstracts suggests that the sample we study is dominated by far by papers that do indeedfocus on Sun and heliosphere, while of course also including topics such as climate forcing, thephysics of the upper atmospheres of planets, comets in the solar wind, cosmic-ray modulation,and weathering of lunar surface materials.

The ADS searches suggest that the productivity of the world-wide community researching theSun and heliosphere continues to grow steadily if measured through its publications (Fig. 1). Arapid growth in the number of refereed publications that started after the Second World Warcontinued up to about 1975. After that, the growth slowed drastically, transitioning to a sus-tained increase that doubles the number of refereed publications on a time scale of approximately40 years, reaching a total of some 2200 refereed publications by 2014.

Such automated searches enable us to process a lot of information, but it is not readilypossible to avoid a distortion of the statistics associated with the author names. For one thing,authors with identical family names and initials for their given names are not differentiated. Also,authors who publish with different spellings or composites of their family names (e.g. marriedand maiden names) or their initials will be counted as separate individuals. These effects biasthe total number of publications and researchers involved, but we expect their impacts to be

SOLAR ACTIVITY 5

Scientific meetings and summer schools

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Figure 5. Number international scientific meetings (solid) and summer schools (dashed) from1997 through 2014 (from the SOHO web site: http://sohowww.nascom.nasa.gov/community/).

Table 2. IAU meetings related to solar activity in the triennium 2012-2014.

2014 IAUS305 Polarimetry: the Sun to stars and stellar environments2013 IAUS302 Magnetic fields throughout stellar evolution2013 IAUS300 Nature of prominences and their role in space weather2012 IAUS294 Solar and astrophysical dynamos and magnetic activity

limited when we review fractional trends over the years, as we do not expect very substantialchanges of the relative impacts of these biases over the years.

The number of unique author names (subject to the above-noted caveat of the way we usedthe ADS system) contributing to refereed publications shows a trend that roughly mimics thatof the number of publications: flat in the first half of the 20th Century with a total populationof merely some 40 active researchers globally, then rapidly growing after WW II, and eventuallygrowing exponentially from about 1975 onward up to a total approaching 5,000. This growth rateof ≈ 2.5%/yr is about twice the growth rate of the world’s overall population (which averagedat ≈ 1.3%/yr over the same period; Population reference bureau 2013), suggestive of a ratherhealthy growth in the research discipline of solar and heliospheric sciences and related fields overthat of the general population.

Many of the authors in the population publishing in the refereed literature are in the fieldonly for a short time, or publish only infrequently as team members on, for example, instrumentand facility papers. If we look for the population of authors who have worked in the field ofsolar-heliospheric sciences long enough to have contributed several papers that have been citeda few times, we can use the so-called H index. That index is computed by ranking all refereedpublications by an author in decreasing order of the number of citations, and then looking forthe rank in that list where rank and number of citations equal. The downward-cumulative Hindex distribution for authors publishing on Sun and heliosphere in 2014 (based on citations inADS, and using refereed as well as unrefereed publications) is shown in Fig. 3.

For the present purpose of selecting researchers with a few years of activity in the field, wetake a rather arbitrary H-index threshold of 5, i.e., looking for authors with at least 5 refereedpublications on their record that have each been cited at least 5 times. Fig. 3 shows that thisincludes about two thirds of all publishing authors in 2014.

The number of publications per year grows somewhat slower than the population of con-tributing authors, so that the number of publications per author shows a steady decrease overthe past century, trending to just under half a refereed publication per year per author (seeFig. 4). That average value is subject to a large range for the population: some authors may notpublish for years in a row, while others may exceed a dozen in particularly productive years, forexample as members of sizable teams working with new instrumentation.

The community exchanges information efficiently at scientific meetings. Fig. 5 shows that the


number of such events tends to increase over the years, with marked fluctuations from year toyear, averaging around 40 meetings per year over the past decade. The Symposia supported bythe IAU through Commission 10 in the last three years are listed in Table 2. Summer schoolsin which new generations of researchers are given broader or deeper views into the community’sactivities appear to grow slowly in frequency, trending towards about five events per year (dashedline in Fig. 5).

Although the scientific community working on “solar activity” appears healthy and growing,there is a clear need to improve how we communicate the excitement about our science to thegeneral public: for example, only 8 in the most recent 400 press releases and news articles listedby the AAS† were related to some aspect of solar activity.

3. Trends in observational capabilities

The observational infrastructure enabling the study of solar physics has seen dramatic ad-vances over the past five years. Among these, we highlight a few:

In 2009, the two STEREO spacecraft (launched into Earth-trailing and Earth-leading orbitsin 2006; Kaiser et al. 2008) passed the quadrature points relative to the Sun-Earth line. Whencombined with Earth-perspective viewing, the following years enabled a view of the entire solarsurface, for the first time in history showing us the evolution of an entire stellar atmosphere.

The Atmospheric Imaging Assembly (AIA; Lemen et al. 2012) on board the Solar Dynam-ics Observatory (SDO - launched in 2010; Pesnell, Thompson, and Chamberlin 2012) providesuninterrupted observing of the outer atmosphere of the entire Earth-facing side of the Sun ata cadence of 12 s and a resolution of close to an arcsecond. Combined with magnetographyand helioseismology with the Helioseismic and Magnetic Imager (HMI; Scherrer et al. 2012),as well as Sun-as-a-star spectroscopy in the EUV with the EUV Variability Experiment (EVE;Woods et al. 2012), this powerful SDO spacecraft sends down well over 1TB/day. The primarydata and higher-level derivatives fill a data archive that now exceeds 7PB and holds over 96%of all data ever taken from space in the domain that focuses on the Sun and the physics of theSun-Earth connections.

Instrumentation with high spatial or spectral resolution is flown on the JAXA-led Hinode/Solar-B mission (launched in 2006; Kosugi et al. 2007) and the NASA Small Explorer IRIS (InterfaceRegion Imaging Spectrograph, launched in 2013; De Pontieu et al. 2014b) that offer images withresolutions between 0.1 and 0.3 arcsec, combined with spectroscopy in the visible and ultravio-let. RHESSI (Lin et al. 2002), launched in 2002, is continuing to provide unique spectroscopicimages of the Sun at high energy.

These space-based instruments provide important access to the domain from photosphere tocorona, critically complemented by ground-based telescopes and their instrumentation, as wellas by rocket experiments such as Hi-C (Winebarger et al. 2014).

In the optical domain, the 1.6m New Solar Telescope (NST, Goode and Cao 2012) at BigBear in the U.S.A. and the 1.5-meter German GREGOR solar telescope at Tenerife in Spain(Schmidt et al. 2012) have been in regular operation since 2012. The 1-m New Vacuum So-lar Telescope (NVST, Liu et al. 2014c) at Fuxian Lake in southwest China is also in regularoperation recently. CRISP (Crisp Imaging Spectro-polarimeter) at the Swedish 1-m Solar Tele-scope (SST; van Noort and Rouppe van der Voort 2008) reached 0.13 arcsec spatial resolutionand high polarimetric sensitivity aided by post-processing. All these telescopes have capacitiesclose to their diffraction limit due to advanced designs and excellent seeing conditions. We haveseen glimpses into very high-resolution ground-based coronal observing as well with, for the firsttime, synoptic observations of coronal Stokes polarimetry in the near infrared by the CoronalMultichannel Polarimeter (CoMP, Tomczyk et al. 2008) telescope.

In the radio domain, the recently commissioned Chinese Spectral Radioheliograph (CSRH;Yan et al. 2009; Wang et al. 2013) at Mingantu in Inner Mongolia of China (renamed as Min-gantu Ultrawide Spectral Radioheliograph-MUSER) is a radioheliograph operating with thewidest frequency range ever reached from 400MHz to 15GHz, with a high temporal, spatial,and spectral resolution. Recent non-solar dedicated radio arrays such as the Murchison Wide-field Array (MWA, Oberoi et al. 2011) and the Low-Frequency Array for Radio astronomy (LO-FAR, Mann, Vocks, and Breitling 2011) have obtained spectroscopic solar imaging at metricand lower frequencies. The recently upgraded Karl G. Jansky Very Large Array (EVLA) has

† http://aas.org/astronomy-in-the-news, accessed on 2015/09/03.

SOLAR ACTIVITY 7

provided solar radio dynamic imaging spectroscopy of type III bursts at decimeter wavelengths(Chen et al. 2013). Even the millimeter domain and beyond is seeing major advances with theAtacama Millimeter/sub-millimeter Array (ALMA; Bastian et al. 2015) observatory coming online for solar observing as it continues to complete its construction phase.

The observations of tens of thousands of Sun-like stars by the NASA Kepler satellite is offeringyet more insights into the physics of the Sun, ranging from an improved understanding of internalstructure and dynamics (with asteroseismic techniques) to a view of rare, extremely energeticflares. Kepler data, with ground-based follow-up studies, suggest that solar flares may occurwith energies up to several hundred times higher than observed directly in the past half centurywith space-based instrumentation (Schrijver and Beer 2014).

4. Trends in IT infrastructure for research and data

The volume of information that needs to be processed by solar researchers is increasing rapidly.In terms of data to be analyzed, we have definitely reached the petabyte era. This is true forobservational data in the archives of SDO, but also in computer experiments in which singlesnapshot data dumps of the advanced codes can exceed a TB.

A tendency towards open data policies means that we have ever more access to large volumesand a daunting diversity of data. That complicates finding, processing, and integrating data.Infrastructure support for, for example, the Virtual Solar Observatory (VSO), SolarSoft IDL(SSWIDL), and the Heliophysics Events Knowledgebase and Registry (HEK, HER) are crit-ically important to enable efficient utilization of the growing data diversity and volume. Thecommunity lags in strategic thinking about these meta-infrastructural elements, both where cur-rent support and future expansion or replacement are concerned. There has also been a recentmovement towards open-source data analysis software, with the development of the SunPy solaranalysis environment in Python (Mumford et al. 2013).

A similar flood of information is found with scientific publications, which exceed 2,000 refereedpublications per year (see above). Here, the support infrastructure of ADS is of critical value. Thewide diversity of journals in which the works of colleagues are published requires subscriptionaccess to many publications, at costs that are increasingly hard to bear for relatively smallresearch groups; here, preprint services such as arXiv and MaxMillennium play significant rolesin making the community aware of what is going on. “Living reviews” as offered by the free on-line journal Living Reviews in Solar Physics enable new researchers to understand the contextof their work and established researchers in one sub-specialty a quick introduction into adjacentareas by their peers.

Among the difficulties the solar activity community also faces is that many solar and inner-heliospheric events are studied by different groups and published in different journals. Findingstudies on a particular solar region of interest is hampered by inconsistent use of the char-acterizing spatio-temporal coordinates of events which may be found in abstract, main text,tables, appendices, and sometimes only marked in figures that are not machine-readable. TheIAU adopted a standard convention for this in 2009 (Leibacher et al. 2010) in its SOL standard(short for Solar Object Locator). Its use is encouraged by journal editors including those of SolarPhysics, the Astrophysical Journal, and the Journal of Geophysical Research. Broad usage ofthe SOL standard would enable computer searches of related publications, enabling researchersto put new (meta-)studies in broader contexts.

5. Trends in research directions and key findings

For the highlights touched upon in this report, we opted for two criteria to identify topics ofinterest. One is to mention specific areas of note in the opinion of the Organizing Committeethat may be new developments, are highly specialized yet significant, may concern new instru-mentation or methods, or are otherwise deemed to be developments that may grow to see moreactivity in terms of publications in the near future.

The other criterion we applied is to be guided towards topics of frequent activity by thecommunity itself by looking at the most cited works. Such a selection does introduce a biastoward the papers published early in the period reviewed, of course, but our purpose is not toidentify the most-cited works per se, but rather to find the dominant themes within the setof these works that apparently resonate strongly within the community already within the 3-ywindow from which they are selected.


We searched for the most-cited refereed publications from ADS with the terms “solar.activity”,“coronal.mass.ejection”, “solar flare”, “solar prominence”, or synonym(s) in their abstracts inthe period 2012-2014. Within this set, we identify the following themes (sorted alphabetically):active-region magnetic fields; coronal thermal structure; coronal seismology; solar flares anderuptions; and the Sun-in-time related aspects of long-term solar variability including cosmic-ray modulation. We close with a collection of developments, discoveries, and surprises.

5.1. Active-region magnetic fields

The photospheric magnetic field of active regions forms the foundation of the overlying atmo-sphere. Its evolution – through emergence, displacement, and submergence of flux – is key todriving eruptive and explosive events in the corona and into the heliosphere. Until recently,generally only line-of-sight magnetic field maps were available for this work (with its routineobservations enabling the study of increasingly large samples; one example of a large study is thework by Al-Ghraibah, Boucheron, and McAteer 2015, who analyse the magnetic properties of∼2,000 active regions looking for signatures likely involved in flaring). Nowadays, regular vector-magnetic determinations from the observed polarization signals are available from sources thatinclude Hinode, SOLIS, and SDO/HMI. The sensitivity of such vector field maps allows thedetection of lasting changes in the photospheric field when comparing pre- to post-flare observa-tions (e.g. Wang et al. 2012). Temporal resolution is so good that coronal events can be tightlybracketed to try to understand the causes of flares and eruptions, as well as the changes in energyand helicity involved; for example, Sun et al. (2012) analyze a series of nonlinear force-free fieldmodels for the evolution in the energy of an active region around the time of a major eruption.

The increasing availability of vector-magnetic data enables statistical studies on the propertiesof active regions and their activity heretofore possible only on line-of-sight magnetograms. Forexample, Bobra and Couvidat (2015) use a data base of vector field maps of over 2,000 activeregions to train a machine-learning algorithm to attempt forecasting of large flares. Su et al.(2014) review 3,000 vector field maps of some 60 regions to compare estimates of free energywith flare rates. Tiwari et al. (2015) analyze a sample of nearly 200 coronal mass ejections toreveal that flux, twist, and proxies for free energy tend to set upper limits to the speed ofCMEs emanating from the active regions studied. Otsuji, Sakurai, and Kuzanyan (2015) reviewhundreds of vector magnetograms of a sample of 80 active regions to study helicity and twistparameters to test for the influence of Coriolis forcing.

One long sought-after goal of flare and CME physics is to use models of the solar atmosphericfield to understand why field configurations destabilize and under what conditions destabiliza-tion begins and proceeds. In a “meta-analysis” review, Schmieder, Aulanier, and Vrsnak (2015)discuss recent developments, including the use of 3D field extrapolations that suggest thattopological structures (notably null points and hyperbolic flux tubes) may be involved in thetriggering and generally as tracers of reconnection processes early on.

But even as the availability of vector-magnetic field maps becomes routine, the realizationis growing that by themselves they appear insufficient to provide generally adequate lower-boundary conditions to understand the solar atmospheric activity. For example, a group ofmodelers using a variety of non-linear force-free field algorithms concludes in a series of studies(see, e.g. DeRosa et al. 2009; Tadesse, Wiegelmann, and MacNeice 2015; DeRosa et al. 2015,and references therein) that snapshot vector-magnetic maps do not suffice to obtain a reliablecoronal field model with accurate energy or helicity measurements, with effects of instrumen-tal resolution and field of view, as well as the model geometry (cartesian vs. spherical) allcompounding the problems. New developments include the use of coronal loop observationsto guide non-potential field models either from a single perspective as is possible currently(e.g. Malanushenko et al. 2014) or by using stereoscopic data from existing or future space-based instrumentation such as STEREO, Solar Orbiter, and SDO (e.g. DeRosa et al. 2009;Aschwanden, Schrijver, and Malanushenko 2015). Methods to follow the evolution of active-region fields based on data driving are also being developed, using the uninterrupted streamof (vector) magnetograms now available from space-based platforms (e.g. Cheung and DeRosa2012; Fisher et al. 2015, based on the MHD-like magnetofrictional approximation), even reachingup to global MHD field models from near the solar surface into the heliosphere (Hayashi et al.2015). Others are developing MHD methods to study CME initiation based on observed surfacefield evolution (e.g. Amari, Canou, and Aly 2014). Fundamental difficulties with these emerg-ing methods include the difficulty in measuring the transverse field in areas of relatively lowflux densities, the intrinsic 180-degree ambiguity in the field direction given a magnitude for the

SOLAR ACTIVITY 9

transverse component, the need to constrain the electric field to drive the model’s evolution, andultimately the quantitative comparison with solar observables to determine the verisimilitudeof the model fields.

Even as our ability to observe and process rapidly growing data volumes on active region fieldsincreases, we remain puzzled by the Sun’s atmospheric magnetic field: we have yet to understandhow large amounts of energy can sometimes be stored to eventually be explosively converted intoflares and CMEs, while in other cases the energy is either not stored or is not explosively released.For a recent review of our understanding of the magnetic field in the solar atmosphere, and thevariety of methods used to observe and study it, we refer to Wiegelmann, Thalmann, and Solanki(2014), and references therein.

5.2. Coronal thermal structure and heating

The X-ray solar corona is made of complex arrays of magnetic flux tubes anchored on both sidesto the photosphere, confining a relatively dense and hot plasma. This optically thin plasma isalmost fully ionized, with temperatures above 1MK, and emitting mostly in the extreme UV toX-rays with intensities proportional to the square of its density. Significant progress was madein studying the physical and morphological features of coronal loops in a series of very successfulEUV and X-ray solar missions, since the first observational evidence of the presence of coronalloops provided by rocket missions in the mid-1960s (Giacconi et al. 1965).

In recent years, several solar missions were launched and started producing spectacular im-ages and data in different spectral ranges. The high spatial and temporal resolution of theseinstruments and the complementarity between these data sets, poses new challenges to under-stand the heating and dynamics of coronal loops. The launch in 2010 of the Solar DynamicsObservatory (SDO, Pesnell, Thompson, and Chamberlin 2012) allows continuous observation ofthe whole Sun with high temporal and spatial resolution with AIA, EVE, and HMI. In partic-ular, AIA images span at least 1.3 solar diameters in multiple wavelengths, at about 1 arcsecin spatial resolution and at a cadence of about 10 seconds. Coronal loops observed by AIAhave been found to be highly variable and highly structured in space, time, and temperature,challenging the traditional view of these loops as isothermal structures and favoring the case formulti-thermal cross-field temperature distributions. Because the thermal conductivity is severelyreduced in the directions perpendicular to the magnetic field, a spatially intermittent heatingmechanism might give rise to a multi-threaded structure in the internal structure of loops. Onepossible example of such an intermittent heating process is MHD turbulence, which is expectedto produce fine scale structuring within loops all the way beyond the resolution of current obser-vations (Gomez, Martens, and Golub 1993; van Ballegooijen, Asgari-Targhi, and Berger 2014).A recent study by Brooks, Warren, and Ugarte-Urra (2012) combining spectroscopic data fromthe EUV Imaging Spectrometer (EIS, Culhane et al. 2007) aboard Hinode and SDO/AIA im-ages, shows that most of their loops must be composed of a number of spatially unresolvedthreads.

More recently, the sounding rocket mission High-resolution Coronal Imager (Hi-C, Cirtain et al.2013), achieved an unprecedented spatial resolution 0.2 arcsec in EUV images. Winebarger et al.(2014) find that the finely structured corona, down to the 0.2” resolution, is concentrated in themoss and in areas of sheared field, where the heating is intense. This result suggests that heatingis on smaller spatial scales than AIA and that it could be sporadic. These results are consistentwith differential emission measure (DEM) analysis that study the distribution of temperatureacross loops. Warren, Winebarger, and Brooks (2012) present a systematic study of the differ-ential emission measure distribution in active region cores, using data from EIS and the X-RayTelescope (XRT) aboard Hinode. Their results suggest that while the hot active region emissionmight be close to equilibrium, warm active regions may be dominated by evolving million degreeloops in the core. More recently, Schmelz et al. (2014) used XRT and EIS data as well as imagesfrom SDO/AIA, and found that cooler loops tend to have comparatively narrower DEM widths.While the DEM distribution of warm loops could be explained through bundles of threads withdifferent temperatures, cooler loops are consistent with narrow DEMs and perhaps even isother-mal plasma. The authors then speculate that warm, multi-thermal, multi-threaded loops mightcorrespond to plasma being heated, while cool loops are composed of threads which have hadtime to cool to temperatures of about a million degrees, thus resembling a single isothermalloop.

The Interface Region Imaging Spectrograph (IRIS, De Pontieu et al. 2014) was launched in2013, and provides crucial information to understand coronal heating, by tracing the flow of


energy and plasma from the chromosphere and transition region up to the corona. IRIS obtainshigh resolution UV spectra and images with high spatial (0.33 arcsec) and temporal (1 s) reso-lution. Recent IRIS observations show rather fast variations (20-60 s) of intensity and velocityon spatial scales smaller than 500 km at the foot points of hot coronal loops (Testa et al. 2014).These observations were interpreted as the result of heating by electron beams generated insmall and impulsive heating events (the so-called coronal nanoflares).

Theoretical models of coronal heating have been traditionally classified into AC or DC, de-pending on the time scales involved in the driving at the loop foot points: (a) AC or wavemodels, for which the energy is provided by waves at the Sun’s photosphere, with timescalesmuch faster than the time it takes an Alfven wave to cross the loop; (b) DC or stress models,which assume that energy dissipation takes place by magnetic stresses driven by slow foot pointmotions (compared to the Alfven wave crossing time) at the Suns photosphere. Although thesescenarios seem mutually exclusive, two common factors prevail: (i) the ultimate energy source isthe kinetic energy of the sub-photospheric velocity field, (ii) the existence of fine scale structureis essential to speed up the dissipation mechanisms invoked (Gomez 2010). For a coronal heatingmechanism to be considered viable, the input energy must be compatible with observed energylosses in active regions, estimated by Withbroe and Noyes (1977) to be ≈ 1× 107erg cm−2 s−1.Welsch (2015) used high-resolution observations of plage magnetic fields made with the SolarOptical Telescope aboard Hinode to estimate the vertical Poynting flux being injected into thecorona, obtaining values of about ≈ 5× 107erg cm−2 s−1, which suffices to heat the plasma.

5.3. Coronal seismology

Coronal seismology, like terrestrial seismology and traditional helioseismology, provides a meansof probing the background state of the medium through which the waves propagate. Althoughthe corona is not hidden from view like the interiors of Earth and Sun, it is very difficultto directly measure plasma and magnetic properties such as density ρ, magnetic field B, ortransport coefficients. Observations of oscillations in the coronal plasma can potentially providepowerful constraints on these quantities, for example through the Alfven velocity B/

√µρ (e.g.,

see the Living Review of Nakariakov and Verwichte 2005).But unlike the solar interior and terrestrial examples where waves are but perturbations on

the background state, the coronal seismic waves are crucially important to the energy balance oftheir host medium. Coronal heating and solar wind acceleration are widely thought to result atleast in part from waves (Ofman 2010). An overview of coronal seismology as of 2012 is providedby De Moortel and Nakariakov (2012).

The last 5–8 years have seen an explosion in coronal wave studies due to the advent ofnew instrumentation, such as the ground-based Coronal Multichannel Polarimeter (CoMP,Tomczyk et al. 2008) and the space-based Atmospheric Imaging Assembly (AIA) on the So-lar Dynamics Observatory (SDO). Both have revealed ubiquitous Alfven-like (i.e., transverseto the magnetic field) coronal oscillations (Tomczyk et al. 2007; McIntosh et al. 2011, respec-tively), though the interpretation of their exact natures – Alfven or kink – is controversial(Van Doorsselaere, Nakariakov, and Verwichte 2008). The term “Alfvenic” is commonly used toencompass both wave types. In either case, the increased resolution of the AIA observations(White and Verwichte 2012) has allowed the detection of sufficient oscillatory power to poten-tially power both the corona and solar wind (McIntosh et al. 2011), though precise mechanismsare not currently known with certainty. The concentration of oscillatory power in the few-minuteperiod band, and in particular at around 5-minutes, strongly suggests a link with the Sun’s in-ternal p-mode global oscillations. Consistency in the estimation of Alfven speed between seismictechniques and magnetic field extrapolation is confirmed by Verwichte et al. (2013) using AIAdata from a flare-induced coronal loop oscillation.

Time-Distance techniques applied to CoMP observations indicate a preponderance of out-ward propagating waves over inward propagation, even in closed loop structures, suggesting insitu dissipation (or mode conversion) on a timescale comparable to the Alfven crossing time(Tomczyk and McIntosh 2009).

Disentangling Alfven and kink waves is addressed in depth byMathioudakis, Jess, and Erdelyi(2013). In the magnetically structured solar atmosphere, the only true Alfven wave is torsional,and there is considerable interest in identifying these in observations because of the amount ofenergy they could potentially contribute to the outer atmosphere. However, being incompressive,Alfven waves are not seen in intensity, and torsional Alfven waves are also difficult to detectin Doppler (De Moortel and Pascoe 2012; McIntosh and De Pontieu 2012). Recently though,

SOLAR ACTIVITY 11

0.33-arcsec high-resolution observations of the chromosphere and transition region (TR) withNASA’s Interface Region Imaging Spectrograph (IRIS), coordinated with the Swedish SolarTelescope, have revealed widespread twisting motions across quiet Sun, coronal holes, and activeregions alike that often seem to be associated with heating (De Pontieu et al. 2014a). This mustpresumably extend to the corona as well.

Dissipation of Alfven waves in the solar atmosphere has long been thought to rely largely onthe generation of Alfven turbulence through nonlinear interaction between counter-propagatingwaves (Cranmer and van Ballegooijen 2005; van Ballegooijen et al. 2011). Observations withCoMP showing enhanced high-frequency power near the apex of coronal loops (Liu et al. 2014a;De Moortel et al. 2014) possibly supports this view.

The presumed acceleration of the solar wind by Alfven turbulent energy deposition poses thechallenge of identifying and explaining counter-propagating Alfven waves in open magnetic fieldregions required to produce that turbulence. Morton, Tomczyk, and Pinto (2015) confirm thepresence of these counter-propagating waves using CoMP. These observations also provide evi-dence of a link to the p-mode spectrum, which presumably relies on magnetoacoustic-to-Alfvenmode conversion occurring in the lower atmosphere (Cally and Hansen 2011; Hansen and Cally2012).

5.4. Flares

5.4.1. The flare’s impact on the lower solar atmosphere

The solar chromosphere is where most of the energy of a solar flare is dissipated and radiated inbright linear structures called flare ribbons, and recent work on this topic has been dominated byobservations from IRIS. The high spectral resolution available with IRIS shows complex spectralline profiles in the Si IV transition region line at 1394A and 1403A sometimes with two or threetime-varying gaussian components within one IRIS spatial pixel (Brannon, Longcope, and Qiu2015). The hot “coronal” line of Fe XXI (1354A) on the other hand shows only a single stronglyblue-shifted component (Brosius and Daw 2015; Graham and Cauzzi 2015) originating in com-pact ribbon sources at the beginning of a flare, with no hot “stationary component” present.The presence of this line also demonstrates the high temperatures reached by the chromospherein flares as deduced from Hinode/EIS observations (Brosius 2013; Graham et al. 2013). WithEIS, we see 1.5-3MK redshifts (Young et al. 2013) confirming earlier reports, as well as signif-icant non-thermal broadening. This means that the momentum-conserving condensation frontthat is produced by flare heating and paired with the evaporation flow contains hot plasma.In turn this implies that the condensation front originates relatively high in the chromosphere,otherwise such high temperatures would not be possible in the standard (electron-beam-driven)model of flares. This is somewhat at odds with recent measurements of element abundance(Warren 2014) which look more photospheric than coronal, suggesting that up-flowing evapo-rated material comes from low down in the chromosphere, below where the normal fractionationby first ionization potential sets in.

Flare optical (or white-light – WL) emission continues to be difficult to observe and diffi-cult to explain. Optical foot points characterized using 3-filter observations with Hinode/SOT(Watanabe et al. 2013; Kerr and Fletcher 2014) could be explained by modest temperature in-creases of the photospheric black body. The other main proposed radiation mechanism is re-combination emission, and flare continuum in the near UV (beyond the Balmer edge) observedwith IRIS has an intensity consistent with this (Heinzel and Kleint 2014), but the tell-taleBalmer jump has not been seen. Co-spatial hard X-ray (HXR) and white-light flare sourceshave been observed using RHESSI and SDO/HMI, and require that both emissions are produceda few hundred kilometres above the photosphere (Martınez Oliveros et al. 2012; Krucker et al.2015), at a height corresponding to the temperature minimum region, and beyond the rangeexpected for HXR-emitting electrons arriving from the corona (unless the chromosphere isunder-dense compared to expectations). It may be possible to generate optical emission fromthe temperature minimum region, for example by modest heating due to ion-neutral damping(Russell and Fletcher 2013), but the presence of non-thermal electrons in this plasma is harderto explain. Also indicating the flare’s impact on the dense lower atmosphere, there have beenmany more reports of “sun quakes” — flare seismic emission (e.g. Alvarado-Gomez et al. 2012;Zharkov et al. 2013) — but the mechanical driver remains uncertain; there are correspondenceswith either HXR sources (pressure pulses from electron-beam driven shocks) or magnetic tran-sients (Lorentz forces) in some but not all cases. High-resolution ground-based flare observationsusing the New Solar Telescope at BBSO show extraordinary fine structure, on a sub-arcsecond


scale, in flare ribbons and footprints (Deng et al. 2013; Sharykin and Kosovichev 2014), settingthe scene for future observations with DKIST.

5.4.2. Magnetic-field evolution and energetics

Examination of the changes in photospheric vector field occurring at the flare impulsive phaseby Petrie (2012, 2013) using the HMI on SDO suggests strong, permanent and abrupt variationsin the vertical component of the Lorentz force at the photosphere consistent with a downward‘collapse’ of magnetic loops, and changes in the horizontal component mostly parallel to theneutral line in opposite directions on each side, indicating a decrease of shear near the neutralline. The brighter the flare (as expressed in the GOES class), the larger are both the total (area-integrated) change in the magnetic field and the change of Lorentz force (Wang, Liu, and Wang2012), although Su et al. (2014) found that indicators of magnetic non-potentiality (e.g. rotation,shear and helicity changes) are more closely associated with flares — see for example the study offlare-associated rotating sunspots by Vemareddy, Ambastha, and Maurya (2012). In the corona,imaging and spectroscopic observations show compelling evidence for plasma flows suggestive ofthose expected around a coronal reconnection region (e.g. Tian et al. 2014; Su et al. 2013).

The growing number of observed flares in archives and the increased coverage of flares inwavelength space and in domains from surface to heliosphere is enabling an improved assessmentof energy budgets. For example, Emslie et al. (2012) quantify energies of an ensemble of large,eruptive flares to find, among others that it appears that the energy in accelerated particlesduring the initial phases of the flare suffice to supply the energy eventually radiated in the flareacross the spectrum, and that that total energy is statistically just under the bulk kinetic energyin associated coronal mass ejections.

5.4.3. Particle acceleration and transport

The central problem in solar flare theory remains the acceleration of the non-thermal electronsrequired to explain observed chromospheric HXR sources. Observations with RHESSI and SDOshow that coronal electron acceleration can be very efficient; Krucker and Battaglia (2014) findthat essentially all electrons in a coronal source of density a few times 109 cm−3 are energized toabove around 10 keV. Kappa distributions, which are found to be a better fit than a standardthermal plus non-thermal distribution in coronal HXR sources (Oka et al. 2015), are shown toarise naturally in an acceleration region when there is a balance between diffusive accelerationand collisions, in the absence of significant escape from the acceleration region (Bian et al. 2014).In the electron-beam model of a flare, electrons must of course escape the corona to producethe chromospheric HXR sources, and the number flux requirements have always been somewhatproblematic. This may be alleviated if electrons are boosted by wave-particle interactions in thecorona; a quasi-linear simulation of coronal electron propagation shows that wave-particle inter-action with the high phase-velocity Langmuir waves generated by density inhomogeneities canaccelerate beam electrons to higher energies, reducing the requirement on electron flux at ener-gies of a few tens of keV by up to a factor ten (Hannah, Kontar, and Reid 2013). Varady et al.(2014) study a model in which electrons are re-accelerated in the chromosphere, concluding thatthis also reduces demands on putative electron beam fluxes (however requirements on the flarechromosphere energy source, not directly addressed in this model, remain the same). Chromo-spheric (re-) acceleration models may also produce electron angular distributions which are moreisotropic, consistent with the angular distributions inferred from inversion of RHESSI mean elec-tron flux spectra, accounting for photospheric X-ray albedo, by Dickson and Kontar (2013). Acompletely different view by Melrose and Wheatland (2014) is that the electron energization inflares takes place in a parallel electric field that develops close to or in the chromosphere, in aregion of anomalous resistivity, if energy is transported Alfvenically — specifically by inertialAlfven waves. However, the orthodoxy remains that energy transport is by electron beams, andthis is now being tested against observations using beam-driven radiation hydrodynamics codes,the output of which can be compared with, for example, IRIS (Rubio da Costa et al. 2015) andEVE and AIA data (Kennedy et al. 2015), so far with mixed success.

5.5. Coronal mass ejections

The understanding of the initiation and evolution of coronal mass ejections (CMEs) has tremen-dously profited from the combination of coronagraphic observations with high cadence imagingin the EUV together with the multi-perspective view provided by the STEREO mission, aswell as from increasingly sophisticated MHD and thermodynamic modeling (for reviews see

SOLAR ACTIVITY 13

Webb and Howard 2012; Aulanier 2014). Magnetic flux ropes play a key role in the physics ofCMEs. But there is a long debate whether flux ropes are pre-existing or formed during the erup-tion. Patsourakos, Vourlidas, and Stenborg (2013) observed the formation of a flux rope duringa confined flare in high-cadence SDO/AIA EUV imagery. Within hours after its formation, theflux rope became unstable and erupted resulting in a CME/flare event. For other CMEs, forexample those associated with quiescent prominence-cavity systems, a variety of observationsindicate a pre-existing flux rope which may erupt bodily as a CME (see e.g. Figure 12 of Gibson2015). Vourlidas et al. (2013) synthesized 16 years of coronagraphic and EUV observations withMHD simulations, and found that flux ropes are a common structure in CMEs; in at least 40%a clear flux rope structure could be identified. In addition, they established a new “two-front”morphology consisting of a faint front followed by diffuse emission and the bright CME leadingedge. The faint front is suggestive of a wave or shock front driven by the CME.

The high-cadence six-passband SDO/AIA EUV imagery allows to perform differential emis-sion measure (DEM) analysis on solar flares and CMEs to study their multi-thermal dynamics(Hannah and Kontar 2012). It was shown that the CME core region, typically identified as theembedded flux rope, is hot (8 – 10 MK) indicative of magnetic reconnection being involved.In contrast, the CME leading front has temperatures similar to the pre-eruptive corona but ofhigher densities suggesting that the front is a result of compression of the ambient coronal plasma(Cheng et al. 2012a; Hannah and Kontar 2013). The hot flux rope is a key indicator to the phys-ical processes involved in the early acceleration phase of the CME (Fan 2012; Cheng et al. 2013).

The environment of CMEs is important for the development of non-radial propagation.Zuccarello et al. (2012) report that during solar minimum conditions CMEs originating fromhigh latitudes can be easily deflected toward the heliospheric current sheet, thus eventuallybecoming geo-effective. Panasenco et al. (2013) showed that coronal holes nearby the CME ini-tiation site can cause strong deflections of CMEs.

Modeling of the initiation of CMEs continues to provide insights into the various forces andmechanisms that may be involved: initiation may involve the kink instability (Kumar et al.2012), sunspot rotation, reduction of tension of the overlying field (Torok et al. 2013), torusinstability (Kliem et al. 2014), and the breakout process (Karpen, Antiochos, and DeVore 2012;Lynch and Edmondson 2013). Which dominates under which conditions and how commonlythese occur remain topics of future work.

5.6. Large-scale EUV waves

Since their discovery by the SOHO/EIT instrument about 15 years ago, there has been a vividdebate about the physical nature of large-scale EUV waves, i.e. whether they are true wavephenomena or propagating disturbances related to the magnetic restructuring due to the erupt-ing CME. In the recent years, there has been tremendous progress in the understanding ofthese intriguing phenomena thanks to the unprecedented observations available, in particu-lar the high-cadence EUV imagery in six wavelengths bands by SDO/AIA combined with theSTEREO multi-point view which allowed for the first time to follow EUV waves in full-Sun maps(Olmedo et al. 2012). There seems now relatively broad consensus that large-scale EUV wavesare often fast-mode magnetosonic waves (of large amplitude or shocks), driven by the stronglateral expansion of the CME (see reviews by Patsourakos and Vourlidas 2012; Liu and Ofman2014). A number of detailed case studies revealed that the CME lateral front and the EUV waveappear originally co-spatial. But when the lateral CME expansion slows down, the EUV wavedecouples from the driver and then propagates freely, adjusting to the local fast-mode speed ofthe medium (e.g. Cheng et al. 2012b; Olmedo et al. 2012). Three-dimensional thermodynamicMHD modeling of well observed EUV waves also supports these findings, showing the outerfast-mode EUV wave front followed by another bright front indicating the CME component(Downs et al. 2012). Statistical studies of EUV waves based on SDO/AIA (Nitta et al. 2013)and STEREO/EUVI data (Muhr et al. 2014) revealed EUV wave speeds that range from closeto the fast magnetosonic speed in the quiet corona to values well above, the fastest ones ex-ceeding 1000 km s−1. Muhr et al. (2014) showed that at least half of the EUV waves understudy show significant deceleration, and a distinct anti-correlation between the starting speedand the deceleration, providing further evidence for a freely propagating fast-mode wave. Theassociation rate of EUV waves with type II bursts, which are indicative of shock waves in thesolar corona, may be as high as 50% (Nitta et al. 2013). Detailed case studies provided a numberof further characteristics suggestive of the wave nature, such as reflection and refraction of EUVwaves at coronal holes and active regions, transmission into coronal holes as well as the initia-


tion of secondary waves by the arrival of the wave at structures of high Alfven speed (Li et al.2012; Olmedo et al. 2012; Shen and Liu 2012; Kienreich et al. 2013) and for one case, Long et al.(2015) have evaluated the EUV wave’s initial energy using a blast-wave approximation, to bearound 10% of that of the associated CME. Liu et al. (2012a) discovered quasi-periodic fast-mode wave trains within a large-scale EUV Wave with a periodicity of 2 min, running ahead ofthe laterally expanding CME flanks. Asai et al. (2012) presented the first simultaneous observa-tions of the propagation of a large-scale EUV wave and an Hα Moreton wave, showing that thewave fronts evolve co-spatially indicating that they are both signatures of a fast magnetosonicwave pulse.

5.7. CME evolution in the heliosphere

The STEREO mission, often in combination with SOHO/LASCO, offers observations of CMEsall the way from their origin on the Sun, and of their propagation in interplanetary space tobeyond 1AU from outside the Sun-Earth line. These data combined with a multitude of otherin-situ space missions have been vividly used to connect remote sensing CME observations tothe field and plasma data observed by in-situ spacecraft, to constrain models of interplanetaryCME propagation, to study CME-CME interaction, and to forecast CME arrival times andspeeds with the ultimate aim improving the prediction of their geo-effectiveness.

Howard and DeForest (2012) and DeForest, Howard, and McComas (2013) tracked a fluxrope all the way from its solar origin to its in-situ signatures at 1AU using the STEREO SEC-CHI EUV imagers, coronagraphs and wide-angle heliospheric imagers. They establish that thecavity in the classic three-part CME is the feature that becomes the magnetic cloud, implyingmaterial ahead of the cavity is piled-up material from the corona or the solar wind.

Modeling of the interplanetary propagation of CMEs makes use of empirical, analytic andnumerical approaches. The analytical “drag-based model” (DBM) is based on the hypothesisthat the Lorentz forces driving a CME eruption ceases in the upper corona and that beyond acertain distance the interplanetary CME (ICME) dynamics is governed solely by the interactionof the ICME and the ambient solar wind plasma (Vrsnak et al. 2013). From the observationalside, a variety of reconstruction methods have been developed and applied to the HeliosphericImager data including one- as well as two-spacecraft (stereoscopic) observations and inclusionof in-situ data and radio type II bursts to better constrain the propagation direction, distanceand speed profile of CMEs in interplanetary space (Rollett et al. 2012; Mostl and Davies 2013;Colaninno, Vourlidas, and Wu 2013; Liu et al. 2013). These efforts result in comparable typicaluncertainties in the CME arrival time of about half a day (Vrsnak et al. 2014; Lee et al. 2013;Mostl et al. 2014).

Studies using STEREO Heliospheric Imager data and in-situ plasma and field measure-ments established that the interaction of CMEs in the inner heliosphere, due to a faster CMElaunched after a slower one, seems to be a common and important phenomenon (Liu et al.2012b; Harrison et al. 2012; Lugaz et al. 2012; Temmer et al. 2012; Mostl et al. 2012; Liu et al.2013; Temmer et al. 2014). The interaction process may cause deflection or merging of CMEs,and either deceleration or acceleration of merged CME fronts (including heating and compres-sion). Liu et al. (2014b), reporting in a fast CME causing an extreme storm, speculate that theinteraction between two successively launched CMEs resulted in the extreme enhancement ofthe magnetic field of the ejecta that was observed in-situ near 1AU.

5.8. Sun-in-time

The unusually deep and temporally-extended activity minimum between sunspot cycles 23 and24, followed by a slowly rising and low amplitude cycle 24, has led to renewed interest in theunderlying causes of solar cycle fluctuations, including Grand Minima. Much attention hasfocused on the so-called Babcock-Leighton solar cycle models, in which the regeneration of thesolar surface dipole takes place via the decay of active regions. Most extant versions of thesedynamo models are geometrically (axisymmetric) and dynamically (kinematic) simplified, yetthey do remarkably well at reproducing many observed solar cycle characteristics (see, e.g.,Karak et al. 2014, and references therein). Explanations for the extended cycle 23-24 minimumand low amplitude cycle 24 have been sought in terms of variations in the meridional flowexpected to thread the solar convection zone (Upton and Hathaway 2014), and patterns ofactive region emergence and associated feedback on surface flows (Cameron et al. 2014; Jianget al. 2015). These successes of the Babcock-Leighton modelling framework have however beenchallenged by helioseismic measurements (Zhao et al. 2013; Schad et al. 2013) indicating that

SOLAR ACTIVITY 15

the meridional flow within the convection zone has a far more complex cellular structure thanassumed in the majority of these mean-field-like solar cycle models. Possible avenues out of thisconundrum are being explored (see, e.g., Hazra et al. 2014; Belucz et al. 2015).

Much effort has also been invested in implementing various form of data assimilation schemesin dynamo models, with the aim of achieving improved forecasting of the amplitude and timing offuture sunspot cycles. At this point in time no existing dynamo model-based forecasting schemehas done significantly better than the known precursor skill of the solar surface magnetic dipolemoment at times of cycle minima, nonetheless progress is likely forthcoming in this area.

Global magnetohydrodynamical simulations of solar convection have also progressed rapidlyin recent years, with many research groups worldwide now running simulations producing large-scale magnetic fields undergoing polarity reversals (e.g. Masada et al. 2013; Nelson et al. 2013;Fan and Fang 2014; Passos and Charbonneau 2014; Warnecke et al. 2014). Due to computinglimitations all these simulations run in parameter regimes still far removed from solar interiorconditions. Nonetheless, many are producing tantalizingly solar-like features, including rota-tional torsional oscillations (Beaudoin et al. 2013) equatorward propagation of activity “belts”(Kapyla et al. 2012; Warnecke et al. 2014; Augustson et al. 2015) cyclic in-phase magnetic modu-lation of convective energy transport (Cossette et al. 2013) and Grand Minima-like interruptionsof cyclic behavior (Augustson et al. 2015). One particularly interesting feature is the sponta-neous production of magnetic flux tube-like structures within the convection zone, as reportedin Nelson et al. (2013). These were found to rise to the top of the simulation domain, partlythrough magnetic buoyancy, while maintaining their orientation in a manner compatible withHale’ polarity laws (Nelson et al. 2014). This has revived the idea that dynamo action could bewholly contained within the solar convective envelope, rather than relying on the tachocline forthe formation and storage of the magnetic flux ropes eventually giving rise to sunspots.

Major efforts have also taken place in reinterpreting and reanalyzing historical observations ofmagnetic activity. Noteworthy in this respect are the analyses of tilt angle patterns for bipolarmagnetic regions (see Pavai et al. 2015, and references therein), and reanalysis of polar faculaedata by Munoz-Jaramillo et al. (2012). Of particular importance is the recently completedrevision of the international sunspot number (SSN) time series. SSN values for the period 1947-present now account for a discontinuity in the manner of counting spot groups having occurredat the Locarno reference station (Clette et al. 2014). Correcting for this leads to a significant (≃20%) decrease in SSN values during the space era. Consequently, reconstructions of solar activityinto the distant past using the SSN as a backbone to extrapolate space-borne measurementswill need to be reassessed.

Radionuclides generated by the atmospheric impact of galactic cosmic rays (GCRs) providea crucial proxy for the evolution in the Sun’s activity (Usoskin 2008; Potgieter 2013) on timescales longer than a few years or a decade, depending on the radionuclide and its depositionin terrestrial natural archives. New ice core data on 10Be and tree ring data on 14C have beencombined to provide better understanding of climate impacts on these records: a joint analysisof composite tree ring data with ice cores from Greenland and Antarctica have enabled theseparation of the common signal (assumed to be dominated by solar and heliospheric variability)from terrestrial variability (Steinhilber et al. 2012). From this, we now have 94 centuries ofdata on a proxy for solar activity. But translating that proxy into details of solar activitythat may drive space weather and terrestrial climate remains a challenge, as reviewed by, e.g.Solanki, Krivova, and Haigh (2013).

5.9. Developments, discoveries, and surprises

And then, of course, there were numerous surprising realizations and discoveries, in the realworld as much as in the rapidly growing virtual world). We mention merely a small samplingin no particular order: a weak solar cycle following an uncommonly low and long solar mini-mum (McComas et al. 2013); a series of X-class flares from AR12192 none of which were as-sociated with a CME, contrasting with statistics to date (Chen et al. 2015; Sun et al. 2015;Thalmann et al. 2015); use of a Sun-grazing comet to probe the high corona and its connec-tion to the innermost heliosphere (Downs et al. 2013; Raymond et al. 2014); an extremely largeamount of dense, cool plasma falling back onto the Sun following a massive filament eruptionproviding a close-up example of distant accretion processes (Reale et al. 2013); reports of enor-mously energetic flares from what would appear to be Sun-like stars (Nogami et al. 2014) andpotential evidence for strong SEP events associated with very energetic solar flaring from 14Crecords albeit without obvious auroral counterparts (Miyake et al. 2012; Usoskin et al. 2013;


Neuhauser and Neuhauser 2015); the successful creation of realistic looking sunspots in thecomputer (Rempel and Cheung 2014); the revision of sunspot numbers that suggests no long-term increase in solar activity occurred over the past few hundred years (Clette et al. 2014);radiative magneto-convective simulations have reached resolution scales of a few kilometers,and suggest comparable energy densities in magnetic and kinetic reservoirs (Rempel 2014);IRIS observations uncovering rapidly-evolving low-lying loops at transition region temperatures,heretofore inferred from emission measure studies but never yet observed (Hansteen et al. 2014);a new model was proposed for coronal heating based on magnetic gradient pumping (Tan 2014);non-potential field models for a continuously-driven corona over a 16-year period was achieved(Yeates 2014); a solar eruption in July of 2012 that would likely have powered a century-levelextreme geomagnetic storm, as for the Carrington-Hodgson flare of 1859, had it envelopedEarth (Baker et al. 2013); the realization that Stokes’ theorem combined with the inductionequation could explain why polar fields should be a good indicator for the strength of thenext sunspot cycle (Cameron and Schussler 2015); the simulation of a sequence of homologousCMEs and demonstration of so-called ”canniballistic” behavior (Chatterjee and Fan 2013); thediscovery of nested toroidal line-of-sight flows and lagomorphic coronal polarimetric signatureswithin coronal cavities indicating the presence of magnetic flux ropes (Bak-Steslicka et al. 2013);rapidly rotating magnetic structures (”magnetic tornadoes”) have been identified, which pro-vide a channel of energy and twist from the solar surface to the corona (Zhang and Liu 2011;Wedemeyer-Bohm et al. 2012); and the X-class flare SOL2014-03-29T made history by becom-ing “the best-observed flare of all time” (according to NASA) as the ground-based Dunn SolarTelescope and the space-based IRIS, RHESSI, and SDO all observed it in detail.

Acknowledgement

We gratefully acknowledge making use of NASA’s Astrophysics Data System in the writingof this report.

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